Crystal growing method



Dec. 11, 1956 A- E. CARLSON CRYSTAL GROWING METHOD Filed Oct. 12, 1953 INVENTOR.

ALLAN E. CARLSON BY w A ORNE United States Patent CRYSTAL GROWING METHGD Allan E. Carlson, Euclid, Ohio, assignor to Clevite Corporation, Cleveland, Ohio, a corporation of Ohio Application October 12, 1953, Serial No. 385,315

8 Claims. (Cl. 23302) This invention relates to the growing of P-type dihydrogen phosphate crystals having piezoelectric properties, particularly primary ammonium phosphate crystals.

Crystals of primary ammonium phosphate (NH4H2PO4) are grown primarily for the purpose of producing Z-cut plates, which are used extensively in electro-acoustic transducers. In a preferred way of growing these crystals, a crystal seed plate is provided whose dimensions perpendicular to the Z-axis are related in a quantitative way to the dimensions of the Z-cut plates to be produced. In this arrangement it is desired that the extent of the crystal in the plane perpendicular to its Z-axis remain substantially unaltered during the growth process. This may be done by restricting the side growth of thecrystal (i. e., growth perpendicular to the Z-axis), which under these circumstances is unproductive of usable crystal material, so that maximum crystal growth may take place in the direction of the Z-axis in order to produce a crystal from which an optimum number of usable Z-cut plates will result.

It is the object of the present invention to provide a novel method of growing P-type dihydrogen phosphate crystals, particularly primary ammonium phosphate crystals, which has provision for restraining the side growth of the crystal, enabling increased growth to take place in the direction of its Z-axis.

In accordance with the present invention, side growth of the crystal is inhibited by the addition to the crystal growing solution in predetermined quantities of certain polyphosphate additives which have been found to have the eflect of restraining side growth without interfering 'with crystal growth in the direction of the Z-axis.

In the drawing:

The figure is a perspective view of a primary ammonium phosphate crystal bar grown in accordance with the method of the present invention.

In the growing of primary ammonium phosphate crystals, successful results have been obtained following the process disclosed in U. S. Patent 2,452,576 to Kjellgren et al., assigned to the same assignee as the present invention. In the Kjellgren et al. process, crystal seeds are placed in a tray which is then filled with a concentrated solution of primary ammonium phosphate heated to a temperature above the saturation temperature of the solution, which is approximately 46 C. The tray is rocked back and forth slowly to cause the solution to flow back and forth across thecrystal seeds and at the same time the solution temperature is lowered gradually to cause the solution to saturate progressively and crystallize on the crystal seeds. After the solution temperature has dropped to about 18 C., which usually takes from 40 to 60 days in commercial growing plants, the grown crystal bars are removed from the trays.

In growing crystals in accordance with the Kjellgren et al. process, at the start of the growing period iron was added to the growing solution, usually in an amount within the range from about .06 gram per liter to .08 gram per liter. As disclosed in the patent, the purpose of add- 2,773,751 Patented Dec. 11, 1956 ice ing iron was to inhibit side growth of the crystals (in the direction of the X- and Y-axes), thereby insuring increased crystal growth in the direction of the Z-axis so as to obtain a greater number of usable Z-cut crystal plates. In addition the presence of the additive reduces spontaneous nucleation, thus allowing a higher degree of supersaturation of the solution than would be practical in the absence of the additive.

The process of the present invention incorporates the general sequence of steps embodied in the Kjellgren et a1. process.

In the present method the crystal seeds of primary ammonium phosphate are placed in trays, after which a concentrated solution of primary ammonium phosphate is poured into the trays, with the solution temperature at pouring being slightly in excess of the saturation temperature for that solution. Each of the crystal seeds is positioned with its Z-axis extending vertically, so that the solution flow back and forth across the crystal, as the tray is rocked about a horizontal axis, is substantially perpendicular to the Z-axis of the crystal, rather than parallel thereto as in the Kjellgren et a1. process. The X and Y crystalline axes extend at 45 angles to the direction of solution flow back and forth across the crystal, so that the prism faces of the crystal are equally exposed to the solution, which contributes to the symmetrical growth of the crystal. In accordance with the present invention there is added to the solution one or more particular polyphosphates in predetermined amounts which have been found in practice to be quite eifective in restricting the side growth of the crystals. These polyphosphates may be added to the solution in the form of any of the sufliciently pure acids of these polyphosphates or water soluble salts of these polyphosphates, of which the sodium salt is undoubtedly the most practical because of low cost, although the corresponding salts of potassium, ammonium or rubidium might be used, if desired. The polyphosphates which have been found to be efiective for inhibiting side growth are those containing the trivalent negative ion P3O9- (trimetaphosphate) or the pentavalent negative ion P3010 (triphosphate). In both of these polyphosphate ions the phosphorous is pentavalent.

The primary ammonium phosphate growing solution at pouring normally has a concentration of about 475 grams of primary ammonium phosphate per liter of solution. The pH of the solution is acidified such that when diluted to 1% concentration by volume (that is, 1 cc. of solution to 99 cc. of water), the resultant diluted solution has a pH of about 4.0. This is for the purpose of preventing or minimizing the precipitation of iron Salts should any iron be present in the solution. With such a growing solution successful results have been obtained by adding to the growing solution, containing no other impurities or additives in significant concentrations, triphosphate (PsOiuions in a concentration within the range from about 0.14 to 0.35 gram of the triphosphate ion (P3O1o-- per liter of growing solution, or 295 to 737 parts of triphosphate ion per million parts by weight of crystal salt in the solution. If the trimetaphosphate (PsO9 ions are to be inserted into the growing solution as the only additive, their concentration should be about 6.4% less, by weight, than those specified for triphosphate, or about 0.13 to 0.33 gram per liter (about 276 to 690 parts per million), assuming substantially immediate hydrolization of the trimetaphosphate in the solution. When both triphosphate and trimetaphosphate ions are added to the substantially pure growing solution as the only additives, the total concentration. of polyphosphate additive ions in the solution may fall .within the range from about 0.13 to 0.35 grain of these polyphosphat ions per liter of solution, or about 276 to 737 parts per million. As the crystal growth progress ses, the polyphosphate is drawn-out of the growing solu-- tion by entering into the crystal lattice and by hydrolytic decomposition so that at the completion of the growing period practically no polyphosphate remains in the solu tion. However, the polyphosphate additive is eifect-ive to inhibit side growth throughout the growing cycle, which indicates a temperatur dependence in the side growth inhibiting capabilities of the polyphosphate additive since smaller concentrations of the additive seem to be effective at the lower temperatures toward the end of the growing period. Experience indicates that some what more side growth takes place for the lower concentrationsof polyphosphat additive within the ranges specified above than for the higher concentrations.

As indicated in the accompanying drawing, the primary ammonium phosphate crystalgrown by the method of the present invention has had very little side growth beyond the dimensions of the original crystal seed in the direction of the X and Y-axes, so that increased growth has taken place in the direction of the Z-axis, enabling an increased number of usable Z-cut plates 11 to be obtained from the crystal.

It is believed that when trirnetaphosphate P3O9 is added to an aqueous solution it hydrolizes and is converted quickly into triphosph ateP3O10 or perhaps a dihydrogen triphosphate, as follows:

If this is true, then the efiectiveness of either of the polyphosphate additions P3O9"- or PaO1o"- in the aqu u p a y am on um pho ph e g n s l n appears to be due in either instance to the presence of the triphosphate ionv (P3010' in the solution.

It is to be understood that the abovespecified range of polyphosphate ion additive concentration is intended for the specific growing solution mentioned, to restrict the side growth to a predetermined amount. In other instances, with somewhat different growing solutions, or where greater or less side growth is desired, the poly? phosphat concentration would be different in order, to accomplish the desired results most effectively. For example, i accordance with the process of the present invention, it is also possible to combine a lower com n ration of. polyp osphate. with iron. as an. add i e for restraining side growth of the, crystals. Thus, with a growing solution having the crystal salt concentration and pH specified above, iron was added in a concentration of 80 parts per million by weight (about .04 am p r ite nd. in addit on. a cla y PO YP Q Phate salt containing the trimetaphosphate ion (P3O1o-- was dded in. a con en ration of 200 par s per millionby weight (about 0.1 gram of polyphosphate salt per liter of solution)- The resulting crystal had very little side growth and resembled substantially that shown in the drawing, except for some rounded end taper immediately below the crown 13, which was due to the presence of iron in the growing solution. The foregoing examples have been described in detail as exemplary embodiments of the present invention without, however, intending that the invention be construed as limited to these specific examples. Routine experimu t o shou be u ficientf r he determinati o hydrogen phosphate crystals other than primary ammoniurn phosphate, such as dihydrogen potassi-umphos- I -phate, although the primary utility of the presentmethod should be in connection with primary ammonium phosphate because here the problem of side growth ismore severe. Also, it is to be understood that the present invention encompasses within its scop the addition to the growing solution for P-type dihydrogen phosphate crystals any polyphosphate ion which upon hydrolysis and/0r depolymerization yields the triphosphate ion (P30l0 such as the hexametaphosphate ion (PO3)6--*, which under certain conditions depolymerizes and ultimately yields the triphosphate ion. Accordingly, while there have been disclosed herein specific preferred embodiments of the present invention, it is to be understood that various refinements, omissions and modifications which depart from the disclosed embodiments may be adopted without departing from the spirit and scope of this invention.

I claim:

1. A method of inhibiting the side growth of P-type dihydrogen phosphate crystals during the growth thereof in a growing solution of such crystals, which comprises the step of providing in the growing solution, in an amount sufficient to .have significant crystal side growth inhibiting effect, polyphosphate falling within the groups consisting of those, containing the ions P309 and PsO1o* 2. In a method of growing a primary ammonium phosphate crystal, the steps of exposing a crystal seed of primary ammonium phosphate to a growing solution of primary ammonium phosphate, and providing in said solution, ina significant amount sufiicient to have substantial efiect in inhibiting side growth of the crystal,

polyphosphate ions falling within the group which consists of P309 and P3Q1o 3. In a method of growing a primary ammonium phosphate crystal, the steps of exposing a primary ammonium Phosphate crystal seed. to an aqueous growing solution of primary ammonium phosphate, and inserting in said solution polyphosphate salt which is readily ionizable in said solution and which contains, in a significant amount. sufficient toinhibit crystal side growth, polyphosphate' ions falling within; the group which consists of P3o9 and P3010 4; A method of growing a primary ammonium phosphate crystal, comprising .thesteps of flowing a supersaturated growing solution of primary ammonium phosphate back and forth across acrystal seed of pri- 7 mary ammonium phosphate, maintaining said solution supersaturated to cause it to crystallize on the crystal seed, and providing in said solution, in an amount greater than a trace amount and sufficient to significantly inhibit side. growth of; the crystal as thesolution crystallizes on the crystal seed, polyphosphate ions falling within the group which consists of P3O and Ps01o' 5. method o ro a pr mary mm n P p at y tal, o p i n t tep o flo i g an aque s gr wing olu io of m -w y ammon um PhQs the, crystal as the solution crystallizes'on the crystal seed,

polyphosphateions falling withinthe group. which consists of PaQe-rand P3010?,"".. V

6., The methodwof claim 5, wherein said solution'has an acidity correspondingto. apH' of: about 4.0.- for a; 1% concentration'thereof in water, .and .the polyphosphate ion concentrationin saicl solutionfalls.within the range from about 0.13 toll-35' gram of polyphosphate ion per liter of solution.

7. The method of claim 5 wherein; said solution has a an aciditycorresponding to a p'Hof about 4.0 for a 1% concentration thereof in=water, the-polyphosphate salt concentration in said solution is about 0.1 gram of polyphosphate salt per literof solution, and there is present 5 in the solution iron in a concentration of substantially References Cited in the file of this patent .04 gram of iron per liter of solution. UNITED STATES PATENTS 8. A method of growing a P-type dihydrogen phosphate 1 901 020 B th M 14 1 33 crystal, which comprises the steps of exposing a seed of 2304850 i 3 the P-type dihydrogen phosphate crystal to a growing 5 237O473 i 5; 1945 solution of the crystal salt, and provldrng in sard so1ut1on 2,452,576 Kjeugren Nov. 1948 polyphosphate ions falling within the group which consists of P309 and P3010 in a significant concentration sufficient to restrain side growth of the crystal. 

1. A METHOD OF INHIBITING THE SIDE GROWTH OF P-TYPE DIHYDROGEN PHOSPHATE CRYSTALS DURING THE GROWTH THEREOF IN A GROWING SOLUTION OF SUCH CRYSTALS, WHICH COMPRISES THE STEP OF PROVIDING IN THE GROWING SOLUTION, IN AN AMOUNT SUFFICIENT TO HAVE SIGNIFICANT CRYSTAL SIDE GROWTH INHIBITING EFFECT, POLYPHOSPHATE FALLING WITHIN THE GROUPS CONSISTING OF THOSE CONTAINING THE IONS P3O9- AND P3O1 